1. Field of the Invention
This invention relates to apparatus and methods for measuring mass flow in a conduit, such as water flow in an irrigation pipe, and relates more specifically to such apparatus and methods which are substantially non-obstructing.
2. Background Art
The desire to measure flow in a conduit has a long history dating back to the time of Caesar and the measurement of the flow of water to householders. More recent developments have led to a variety of applications for devices measuring flow in a conduit.
The need to measure fluid flow in a conduit may arise from a desire to control, track, or adjust the amount of fluid being delivered through the conduit. Naturally, measuring the flow in a conduit is useful in a number of applications. One such application is measuring the flow of water through a sprinkler pipe, particularly in agricultural irrigation applications. This is desirable for several reasons, including the desire to track and control the amount of water delivered to a given tract of land and the time over which the water is delivered, so that adequate irrigation for the crop being grown is provided. Additionally, in regions where irrigation is needed for growing crops, water is usually a precious commodity and, therefore, the efficient use of water is highly desirable. For such reasons, irrigation systems require the ability to monitor the delivery of water and measure and control the flow rate.
A number of devices for measuring flow rate exist for various applications. The size of the conduit being used, accuracy, cost, and other factors all play a role in determining what type of measuring device will be used for a specific application. One of the most widely used type of device is the so-called differential pressure producing flowmeter. The principle on which this type of device operates is that when the flow in a conduit is contracted (or squeezed), kinetic energy increases at the expense of available potential energy. A feature, therefore, in existing devices for measuring flow in this fashion is to contract the flow through the conduit. Typical systems for reducing the flow include installing a section of pipe which tapers to a significantly smaller diameter, inserting a blockage in the conduit, or creating some other obstruction.
As will be appreciated, contracting the flow of water through a sprinkler pipe is undesirable for a number of reasons. For example, irrigation water often contains debris which can become lodged in a small diameter pipe or caught on an obstruction. This can result in plugging of the pipe, requiring time, energy, and expense to unplug or otherwise repair it. In addition, serious incidents of plugging or damage may jeopardize crops which go unwatered during the time spent unplugging or repairing the pipe. This is particularly true during critical periods in a crop's growing cycle.
An additional problem with differential pressure producing devices currently available is that there is often significant retrofitting required to incorporate them into the system where flow is being measured. For example, in the case of devices which use a gradual reduction in the diameter of the conduit, a relatively long section of conduit must be removed and replaced with a tapering conduit section.
Yet another problem with current devices for measuring flow in a conduit is that variations in temperature and humidity can adversely affect their operability and accuracy. This is particularly true if the variations in temperature or humidity are pronounced. Unfortunately, these are often exactly the types of conditions encountered in agricultural irrigation applications in arid regions. Arid regions can experience wide variations in temperature with hot days and cold nights. In addition, the irrigation systems themselves may cause variations in humidity.
Another prior art approach to measuring flow rate is the so-called elbow flow meter in which a curved section of pipe (the elbow) in the fluid delivery system is fitted with pressure sensors to measure pressure differential in the elbow. In order to measure the flow accurately, the sensors must be precisely placed in both the outer and inner circumferential walls of the elbow, in the same radial plane, and then must be calibrated. (See J. W. Murdock al., "Performance Characteristics of Elbow Flow Meters," Trans. of the ASME, September 1964.)
It would be an advantage in the field of flow measurement to provide a differential pressure measuring device and method which would be relatively simple to install, substantially accurate through differing temperature and humidity ranges, and substantially non-obstructing such that the likelihood of plugging of the conduit in the area of the device is lessened. It would also be an advantage to provide such a device and method capable of measuring backflow and, if desired, taking action based on the occurrence of backflow. Further, for multi-phase flow measurement, it would be advantageous to track density and density changes in the fluid.